Social influences on well-being in squirrel monkeys are being evaluated in animals housed with like-sex and/or opposite-sex companions. The assessment of well-being assumes as major criteria the ability to carry out basic biological functions in a normal fashion and the absence of persistent symptoms of stress or disease. Accordingly, the impact of social living arrangements on well-being is being evaluated using the following measures 1. Activity and reactivity in stress responsive systems, 2. Reproduction, and 3. Health status. Social living arrangements alter pituitary-adrenal activity and seasonal changes in gonadal hormone production and reproductive success. For the first several months following group formation, males and females living with both same sex companions exhibit a sustained reduction in basal adrenocortical activity. It appears that this reduction may reflect instability in relationships and may be inimical to health. Adrenocortical activity increases seasonally, and the timing of this seasonal change is more strongly synchronized socially than are gonadal hormone changes. The presence of multiple females enhances reproduction and gonadal hormone activity in males and females. Surprisingly social facilitation of reproduction is most pronounced in socially subordinate females. Social tension among adult squirrel monkeys is greatest in the presence of young squirrel monkeys, suggesting that irritation or frustration rather than competition is the more common source of aggression in this species. *KEY*Pituitary adrenal activity, Social influences, Reproduction, Pituitary-gonadal activity, Aggression